Physical Sciences Division Research Highlights

Building the Ideal Rest Stop for Protons

When protons rest on the metal-bound dinitrogen groups (green), this is the desired location for producing ammonia or NH3. If the protons rest on the metal center (red), the reaction halts. Subtle differences in the amine groups (blue) greatly change where the protons end up. Understanding the motion of protons is critical to understanding and then designing catalysts for energy and chemical production. Enlarge Image.

Results: Where
protons, or positive charges, decide to rest makes the difference between
proceeding towards ammonia (NH3) production or not, according to
scientists at Pacific Northwest National Laboratory (PNNL) and Villanova
University. By designing and examining the reactivity of five complexes with molybdenum
metal centers, the team found that subtle differences in the complexes greatly
change where the protons end up. The differences were in the structure of the
ligands, molecular frameworks that surround the metal. When the ligands were more
willing than a metal-bound dinitrogen group to take in a proton, the protons
ended up binding with the molybdenum center, essentially ending up stuck in the
wrong place. But, when the ligands ability to accept protons was more closely matched
with that of the dinitrogen group, the protons ended up going to the desired
location for producing NH3.

Why It Matters: Renewable energy. Available food. Effective medicines. The reactions
behind these products and countless more are not as efficient or effective as
we want. Too much waste. Too much energy. Or simply not possible with today’s
chemistry. To improve the reactions, scientists must first understand and then design
catalysts, the molecules that drive reactions without being consumed by them. Specifically,
scientists need to know how catalysts move protons, so they are delving into
one reaction – ammonia production from dinitrogen molecules – that is a six
proton, six electron process. Understanding the in’s and out’s of how the complexes
react with protons could lead to more efficient catalysts.

“This study is
leading us toward optimizing complexes to facilitate difficult reactions by
giving us details about how altering the molybdenum compounds ligand structure will
influence their reactivity,” said Dr. Michael
Mock, who leads the nitrogen studies at the Center for Molecular
Electrocatalysis.

Methods: The researchers
are studying the conversion of dinitrogen molecules to ammonia and are focused
on understanding how, where, and why the 6 protons involved in the reaction
move. In designing the metal-based scaffolding to study the first step in
ammonia production, the team was inspired by both natural processes and
industrial processes that produce ammonia. Natural processes, which are catalyzed
by nitrogenase enzymes, proceed at ambient temperature and pressure. In
industry, the Haber-Bosch process
uses a heterogeneous catalyst and produces millions of tons of ammonia every
year under conditions using high temperatures and pressures.

“Our work is
inspired by these processes, and we are striving to understand proton movement
from a molecular perspective in this challenging multi-proton, multi-electron reaction,”
said Labios.

The team took on the
first step in producing ammonia: forming
a molybdenum-hydrazido intermediate (Mo-NNH2). They tested five different
compounds with different ligands bound to molybdenum at a low oxidation state,
Mo(0). Each molecule had different ligands containing different amine groups as
proton relays, which are basic sites designed to facilitate proton movement in
the presence of acid to the metal-bound dinitrogen group. Using nuclear
magnetic resonance spectroscopy, in situ infrared spectroscopy, and their
expertise with proton motion, they determined the protonated products that were
formed.

In complexes where
the ligands and the dinitrogen groups did not have similar pKa values, measures of their
capacity to donate protons, the reactions produced molybdenum-hydride compounds,
in which the protons were attached to the molybdenum center. This molecule is
not readily converted into the desired molybdenum-hydrazido intermediate, Mo-NNH2.
Further, certain ligands resulted in complexes that were not highly soluble,
limiting their ability to work in the reaction’s solutions.

When the pKa values between the ligands
and the dinitrogen groups were more closely matched, the reactions generated a greater
ratio of the molybdenum-hydrazido triflate complex, the desired precursor.

What’s Next? The researchers in the Center for Molecular Electrocatalysis are more
closely studying the structural influence of the proton relays. Their results
could lead to gaining further insight into how to match the pKa values and optimize the
role of the proton relays in these complexes. They are also screening an array
of molecules in the presence of protons and electrons to find one that can
serve as a catalyst to produce ammonia.